In the oil and gas industry, bores are drilled to access subsurface hydrocarbon-bearing formations. The bores are drilled using bits forming parts of bottom hole assemblies (BHAs) mounted on the ends of strings of drill pipe. The drill pipe string comprises a number of drill pipe lengths which are stored on surface as “stands”, each stand comprising three lengths of drill pipe. Each length of drill pipe comprises a relatively thin-walled central portion and much thicker-walled end connectors which define smaller internal diameters (typically 3-3½″) and larger outer diameters. As a drill string is advanced into a bore new stands of pipe are added to the upper end of the string.
The drilled bore is typically filled with drilling fluid or mud. Thus, as the drill string is made up and lowered into the bore, the fluid surrounds the drill pipe. The majority of drill strings are adapted such that the fluid may flow into the hollow string as it passes into the fluid-filled bore, that is the string is “self-filling”. However, in some circumstances the drill string is not self-filling and must be top-filled, due to the presence of a non-return valve in the string: in the absence of fluid in the string, the external hydrostatic pressure may reach a level sufficient to crush or collapse the hollow drill string.
Conventionally, a drill string will be top-filled after ten drill pipe stands have been run into the bore. This is achieved by trickling drilling fluid into the open upper end of the string. This top-filling operation is tricky and potentially messy, with spillage of drilling fluid being common. Also, top-filling ten stands of pipe typically takes between five and ten minutes, and additional time will often be required to clean-up any spills.
The drilled bore is subsequently lined with tubing, known as casing, which must be top-filled as it is run into the bore. However, top-filling casing is generally far easier than top-filling drill pipe, due to the significantly larger internal diameter of the casing (typically 8½-17½″), facilitating flow of the displaced air from the casing. Also, the internal diameter of casing is constant over the length of the casing string and does not feature any internal restrictions.
The subsequent bore-lining step involves running tubing, known as liner, and suspending the liner from the lower end of the casing. The liner is run into the bore on drill pipe and is provided with a non-return valve at its leading end. Thus, the liner and supporting drill pipe string require top-filling.